Voltage controlled field-effect transistor l-c oscillator



Jan.'7, 1969 J. J- BOYAJIAN 3,421,111

7 VOLTAGE CONTROLLED FIELD-EFFECT TRANSISTOR L-C OSCILLATOR Filed Aug.29, 1967 Sheet of 2 22 32 e2 25 52 5e 64 4e 5 k L x aus I CC ZIi' 29 $243 I C v C(v) c 20 E 42 W M 1 V 3o cc 58 J i.- 5 4 4a FIG. I.

C C(v) l 7. L? Y R m I I I FIG. 2.

INVENTOR.

' JOSEPH J. BQYAJIAN BY ROY MILLER ATTORNEY. GERALD F. BAKER AGENT.

Jan. 7, 1969 I J, BQYAJIAN 3,421,111

VOLTAGE CONTROLLED FIELD-EFFECT TRANSISTOR L-C OSCILLATOR Filed Aug. 29,1967 v Sheet 2 012 290 I II Q 240 Z LU D O Lu m 230 LI- FIXED ems =-|.5"FOR cmcurr OF FIG.

2l0 s'rmeu'r use ACTUAL RESPONSE I I l I I l l I 0 L0 2.0 3.0 4.0 5.06.0 v 7.0 GATE VOLTAGE (v VOLTS FREQUENCY VERSUS VOLTAGE RESPONSE FIG.3.

. INVENTOR. JOSEPH J. BOYAJIAN,

BY ROY MILLER ATTORNEY. GERALD F. BAKER AGENT.

United States Patent f 1 Claim ABSTRACT OF THE DISCLOSURE The linearrange of a voltage controlled oscillator is extended through the use oftwo or more voltage controlled elements in the VCO circuitry. In anexemplary oscillator circuit a field-effect transistor serves the dualfunctions of active element and voltage controlled element, and avaractor diode serves as a second voltage controlled element.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

Background of the invention The present invention relates to oscillatorsand more particularly to voltage controlled oscillators and specificallyto a voltage controlled oscillator having a wide tuning range, highstability, and a linear tuning response.

Prior art oscillators whose frequency may be varied electronically havebeen used for frequency modulators, automatic frequency controlledloops, and sweep frequency generators, for example. The electronicallyvariable oscillators in use are generally the reactance tube controlledoscillator, the RC oscillator, and the multivibrator. Prior to thisinvention it has been observed that the available voltage controlledoscillators at best had a maximum linear operating range of only about(i.e., 10% of the VCO center frequency).

According to the present invention, means is provided whereby the linearrange (i.e., frequency change as a function of voltage change) of avoltage controlled oscillator (VCO) may be extended. As will bedemonstrated, the linear range of a VCO can be extended by using two ormore voltage controlled circuit elements in a manner such that theyreduce each others non-linearities. A voltage controlled oscillatorbuilt according to the invention has been shown to achieve a 30% linearrange (i.e., 30% of center frequency). Any voltage controlled circuitelement may be used.

Brief description of the drawings FIG. 1 shows a schematic of a linearvoltage controlled oscillator according to the invention;

FIG. 2 is a low frequency equivalent circuit of the VCO of FIG. 1; and

FIG. 3 shows a linearity chart of the oscillator of FIG. 1.

Detailed description of the invention Illustrated in FIG. 1 is anoscillator which represents a practical and preferred embodiment of theinvention. A controlled voltage is applied to the oscillator at terminal20. In series connection between terminal 20 and junction 30 are thefixed inductance L and a fixed resistance R From junction 30 the voltagepotential is applied to the gate of field effect transistor 50 and to avariable capacitance device or varactor 60. The element 60 may be anydevice which exhibits usable variations in capacity at a first impedancelevel in response to variations in current or voltage of a first rangeof magnitude and exhibits a sub- 3,421,111 Patented Jan. 7, 1969 icediode which is responsive to variations in reverse bias to exhibitvariations in capacitance at a high impedance level and responsive to aforward bias to exhibit a very low impedance. Bias for varactor 60 issupplied through terminals 22, 24 with a choke coil 32 located betweenterminal 22 and the varactor 60. The second voltage variable element inthis circuit is the field effect transistor 50. The field effecttransistor is a three-termial semiconductor device with high input andoutput impedances (measured in the megohms), and a transconductance gwhich is typically of the order of 1,000 nmhos. The g is a fuction ofthe voltage of the controlled element (which is denoted the gate), andmay be varied over a wide range, (such as 10* to 1).

The power supply for the oscillator is located at V connected acrossterminals 42, 44. An inductance L is inserted between terminal 42 andjunction 64. Regulation of the VCOs output is obtained by a diodenetwork 40. Coupling capacitors 25, 27 and 29 are inserted in order toseparate A-C and DC current components in the circuits. The resistance Rrepresents the load and is connected across output terminals 46, 48.

To better understand why the oscillator according to the inventionoperates as it does, we will first consider the circuit elementconstraints necessary for oscillation and secondly consider themathematical analysis of a VCO built to the constraints and having twovoltage variable elements which are controlled by a single voltagesource as illustrated in FIG. 1.

The oscillator of FIG. 1 is of the class wherein a parallel resonantcircuit is shunted by a negative resistance that has an absolutemagnitude less than the parallel resonant impedance of the circuit.

FIG. 2 shows a current source g e which is representative of PET 50shown in FIG. 1. The current source is placed in parallel with theseries combination of L R and C(V). The current source will vary inaccordance with the voltage appearing across R and L FET 50 is chosen sothat its g which is voltage controlled, will exhibit a negativeresistance characteristic in combination with L and R The amount ofnegative resistance is responsive to gating voltage V Thus a parallelresonant circuit is created by the parallel combination of R L and C(V)shunted by some negative resistance created by the combination of PET50, L and R As gating voltage V is applied to FET 50, oscillations willstart up. As gating voltage V is increased, the frequency of oscillationwill increase as shown in FIG. 3.

Circuit constraints For an oscillator, Equation 1 becomes *E. S. Kuh andD. 0. Pederson. Principles of Circuit SynlthBSiSf lVICGl'dW-Hill Book00., Inc., 1959, p. 52.

det. [Y]

Performing the necessary operations we now find the constraints oncircuit element values such that Equation 3 is satisfied.

g R min. (6)

C(v) 'rnax.=L R (7) Thus, given any two elements (e.g., f and g thevalues for other circuit elements are determined.

Let

C(v)=C min.( (9) where C =capacity of varactor 60 when V= V V =breakdownvoltage of varactor 60 -i =contact potential of varactor 60 V=reversebias applied across varactor 60 and where g =transconductance of PET 50when V =0 V gate voltage of PET 50 W ==pinch 01f voltage of PET 50.

Substituting Equations 9 and 10 into Equation 8, We have Since 1 is tobe a function of one variable V let V=:V V where Vbias is constant butcan be plus or minus.

Then Equation 11 becomes 1 where Applying the binomial theorem toEquation 12, we have 4 for all A A such that l (A V and 1 (A V Equation8 is approximated by where Examining Equation 14, we can point outseveral interesting facts.

(1) The frequency, f, can increase or decrease as V increases dependingon polarity of Vblas (i.e., if the capacitance increases or decreaseswith V (2) If we can make the coefficients of V (for all n 1) equal tozero then f will be a linear funtion of V (3) The VCO will have agreater range of linear response for positive rather than negativevalues of A 1+ 2) 1 2) (4) If Equation 13 has a number of different A(n: l, 2, 3 then there exists the possibility of making the coefiicientsof each Vg (for 11:2, 3, 4 equal to zero. That is to say, the greaterthe number of voltage variable elements we can have, then the closerEquation 13 will approach Equation 14.

Experimental results Using in the circuit of FIG. 1 a 2N3823 fieldeffect transistor (PET) and a TRW PC 134 Varicap brand voltage variablecapacitor, We find the values of the rest of the circuit elements fromEquations 4 to 8 to be and For V =3 v. the frequency should be kc.,however, the measured frequency was 240 kc. as shown in FIG. 3. Thelarge error was caused by using which is not true. The approximate valueof R with R =20K, is 5K. Using R =SK the frequency from Equation 8 is280 kc.

The performance of this VCO is illustrated by FIG. 3 and it is seen thatthe response is fairly linear over a 30% range.

Conclusions- Equation 13 shows that by using two or more voltagevariable elements in a VCO, its linear range of operation can beincreased.

In practice, it is easier to empirically derive the required biasvoltage than it is to solve Equation 13 due to difficulty in arriving atexact equations for g (v) and C(v).

Obviously may modifications and variations of the present invention arepossible in the light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claim the invention maybe practiced otherwise than as specifically described.

What is claimed is:

1. An oscillator circuit comprising:

a field effect transistor having source, drain and gate electrodes;

means for grounding said drain electrode;

means for placing said source electrode at a first higher potential thanground to bias said field effect tran- 5 6 means for placing in parallelwith said load resistance References Cited the series combination of:UNITED STATES PATENTS a coupling capacitance; and

a backtmback diode network; 3,281,699 10/1966 Harwood 331-117 X meansfor connecting a voltage variable capacitor, a 5

varactor, from the gate electrode to the source elec- OTHER REFERENCEStrode of said field effect transistor; Crystalonics, Inc., Detuning andTemperature Commeans for establishing a gating potential; pensation ofthe Varactron Diode, application notes,

means for connecting the series combination of a gating ANV-ll,September 1965, pp. 1-2. 331-36C.

resistance and inductance from said gating potential 10 to the gatingelectrode of said field effect transistor ROY LAKE, Primary Examinertoinduce oscillation in the voltage appearing across SIEGFRIED GRIMMAssistant Examiner said load resistance; and

means for varying the gating potential which in turn CL changes thepotential applied across said voltage vari- 15 able capacitor forchanging the frequency of oscilla- 331 1O9 183 tion of the voltage whichappears across said load resistance.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,421,111 January 7, 1969 Joseph Jacob Boyajian It is certified thaterror appears in the above identified patent and that said LettersPatent are hereby corrected as shown below:

Column 3, lines 55 and 56, the formula should appear as shown below:

Signed and sealed this 31st day of March 1970.

(SEAL) Attest:

EDWARD M.FLETCHER,JR.

WILLIAM E. SCHUYLER, JR, Attesting Officer Commissioner of Patents

